The Iraqi ambassador to Washington on Tuesday predicted a gradual reduction of U.S. troops there over the next few years, but says the biggest obstacle to withdrawal is the lack of a strong Iraqi security force.

One problem, ambassador Samir Sumaida'ie said, is that "criminals" have to be weeded out. As Interior minister four years ago, the ambassador said he had to force 20,000 out of Iraq's security service.

"Improvement in Iraqi security is the surest route to withdrawal," Sumaida'ie said at a Washington think tank as Gen. David Petraeus, the top U.S. commander in Iraq, was testifying before the Senate Armed Services Committee.

Petraeus called for an open-ended suspension of U.S. troop withdrawals this summer. There currently are 160,000 U.S. troops in Iraq, and the Pentagon has projected that when the scheduled troop withdrawals are completed in July there will be about 140,000 troops there.

"We urge America to gradually turn the burden over to us," Sumaida'ie said at the Center for Strategic and International Studies. He spoke confidently of Iraq making progress in protecting itself and deterring any effort by Iran to make inroads in the troubled country.

"Iraqis are ready to resist," he said. "There is no willingness on the part of Iraqi Shiite leaders to work as surrogates for Iran. The Shiites of Iraq are very Iraqi."

"I predict a gradual reduction of American forces over the next few years," the ambassador said, suggesting that the U.S. presidential candidates calling for a pullout - Democrats Hillary Rodham Clinton and Barack Obama - could have a different view as president.

At a news conference, Sumaida'ie, like ambassadors generally, said he preferred to stay away from the politics of the election campaign. But he went on to say that whoever is elected "will have to look at the reality of the situation." Without referring by name to Clinton, Obama or John McCain, the probable Republican nominee, the ambassador said that "when in the White House the candidate will have a different mindframe."